Petroleum coke, used to produce carbon and graphite articles such as electric furnace electrodes, is calcined by heating to a temperature above 1200.degree. C. to remove volatile components and to densify the coke. Thereafter this calcined coke is mixed with a carbonaceous binder pitch, formed into the shape of the article and carbonized by heating to a temperature above 800.degree. C. Where graphitization is required, the article is further heated to temperatures of at least 2800.degree. C.
Petroleum coke particles have a tendency to "puff", that is, to expand and even to split when heated to temperatures above 1500.degree. C., if they contain more than about 0.5 percent by weight sulfur. Electrodes made from such cokes expand, lose density, lose strength, and sometimes split lengthwise when heated to these temperatures. Carbon electrodes which are not graphitized during the manufacturing process may nevertheless reach temperatures between about 2000.degree. C. and 2500.degree. C. during their use in silicon or phosphorus furnaces.
Puffing is associated with the release of sulfur from its bond with carbon inside of the coke particles. If the sulfur-containing vapors cannot escape from the particles or from the electrodes fast enough, they create internal pressure which increases the volume of the particle and may cause the electrode to split.
The conventional remedy for puffing has been to add iron oxide which acts as a puffing inhibitor to the coke-pitch mix before the electrodes have been formed. It has been common practice to add up to two (2) weight percent iron oxide to the mix to reduce coke puffing. Some cokes that have a higher tendency to puff or start puffing at a lower temperature cannot be controlled by iron oxide adequately for fast graphitization.
A recent improvement in the treatment of petroleum cokes to inhibit coke puffing is disclosed in U.S. Pat. No. 4,875,979, issued Oct. 24, 1989 to Orac et al., the disclosure of which is hereby incorporated by reference. This patent discloses use of the improved treatment in a horizontal rotary calciner, which treatment comprises contacting the particles of the high sulfur petroleum coke with a compound containing an alkali or alkaline earth metal selected from the group consisting of sodium, potassium, calcium and magnesium, at an elevated temperature above that at which the alkali or alkaline earth metal compounds begins to react with carbon, but below the temperature at which the coke particles would begin to puff in the absence of the compound. The coke particles and inhibitor compound are maintained at the elevated temperature for a sufficient period of time to permit the reaction to proceed and allow products of that reaction to penetrate the particles and form an alkali or alkaline earth metal containing compound throughout the mass of the particles. According to the disclosure of U.S. Pat. No. 4,875,979, the inhibitor is added to the coke particles either in a separate reactor vessel located downstream from the horizontal rotary calciner and upstream from the cooler, or in a hot zone at the inlet of the cooler especially formed by locating a circular refractory ring in the entrance portion of the cooler and moving the quench water spray nozzle downstream from the refractory ring. The so-treated coke particles are thereafter cooled in the conventional cooler used with rotary calciners.
Although the process of U.S. Pat. No. 4,875,979 is quite useful in the manner disclosed, some problems occur in attempting to adapt the treatment process to existing rotary hearth calciners which rotate about a vertical axis. Some of such existing rotary hearth calciners are equipped with indirect water sprayed coke coolers that do not have internal refractory lining and therefore cannot accommodate an internal hot reactor. If one were to attempt to apply the treatment of U.S. Pat. No. 4,875,979 to such a calciner, either the entire cooler would have to be replaced or a separate reactor vessel would have to be supplied between the discharge outlet of the rotary hearth calciner and the cooler. Both alternatives are very expensive. Another possibility would be to add the inhibitor to the raw petroleum coke entering the calcining hearth. However, the gas flowing above the hearth of the calciner could pick up part of the inhibitor and carry it away, thereby making it unavailable for reaction with the coke. This would also result in undesirable air pollution.
In view of these potential problems, it is a primary object of the present invention to provide an apparatus for treating petroleum cokes with a puffing inhibitor adapted for use in a rotary hearth calciner.
It is another object of the present invention to provide an apparatus for adding puffing inhibitor to petroleum cokes in a rotary hearth calciner which minimizes the loss of inhibitor to the waste gas stream.
It is a further object of the present invention to provide an apparatus for adding puffing inhibitor which may be easily and inexpensively retrofitted to existing rotary hearth calciners.
It is yet another object of the present invention to provide an apparatus for adding puffing inhibitor in a rotary hearth calciner which is durable yet inexpensive to operate.